Synergistic effect of intra- and intermolecular hydrogen bond in 2-haloethanols probed by infrared

Abstract

Fluorine is often considered the only halogen to effectively engage in hydrogen bonds, while the other halogens, particularly iodine, are not electronegative enough to participate as hydrogen bond acceptors in electrostatic interactions. 2-Fluoroethanol and 2-iodoethanol have been studied herein to test this assumption, since a highly stable gauche conformation can experience the intramolecular hydrogen bond. However, the infrared O single bond H stretching frequency indicates that the hydroxyl group in 2-fluoroethanol is not engaged in intramolecular hydrogen bond, while the corresponding vibration mode for 2-iodoethanol suggests that not only the O single bond H is engaged in such interaction, but also that intramolecular hydrogen bond may drive the conformational equilibrium in this molecule. Theoretical calculations support the covalent nature of this interaction, and provide evidence that intermolecular hydrogen bond with a water molecule, and probably with the polar solvents tested experimentally, occurs with the hydroxyl rather than with the iodine substituent, as conventionally, in order to keep the intramolecular hydrogen bond effective.